DGS-3630 Series Layer 3 Stackable Managed Switch Web UI Reference Guide
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The third type of router created in the PIM-SM configuration is the Boot Strap Router (BSR). The goal of the Boot
Strap Router is to collect and relay RP information to PIM-SM enabled routers on the LAN. Although the RP can be
statically set, the BSR mechanism can also determine the RP. Multiple Candidate BSRs (C-BSR) can be set on the
network but only one BSR will be elected to process RP information. If it is not specified which C-BSR is to be the
BSR, all C-BSRs will emit Boot Strap Messages (BSM) out on the PIM-SM enabled network to determine which C-
BSR has the higher priority and once determined, will be elected as the BSR. Once determined, the BSR will collect
RP data sent from candidate RPs on the PIM-SM network, compile it and then send it out on the LAN using periodic
Boot Strap Messages (BSM). All PIM-SM Routers will get the RP information from the Boot Strap Mechanism and
then store it in their database.
Discovering and Joining the Multicast Group
Although Hello packets discover PIM-SM routers, these routers can only join or be “pruned” from a multicast group
through the use of Join/Prune Messages exchanged between the DR and RP. Join/Prune Messages are packets
relayed between routers that effectively state which interfaces are, or are not to receive multicast data. The frequency
at which these messages can be sent out on the network can be configured and are only valid to routers if a Hello
packet has first been received. A Hello packet will simply state that the router is present and ready to become a part of
the RP’s distribution tree. Once a router has accepted a member of the IGMP group and it is PIM-SM enabled, the
interested router will then send an explicit Join/Prune message to the RP, which will in turn route multicast data from
the source to the interested router, resulting in a unidirectional distribution tree for the group. Multicast packets are
then sent out to all nodes on this tree. Once a prune message has been received for a router that is a member of the
RP’s distribution tree, the router will drop the interface from its distribution tree.
Distribution Trees
Two types of distribution trees can exist within the PIM-SM protocol, a Rendezvous-Point Tree (RPT) and a Shortest
Path Tree (SPT). The RP will send out specific multicast data that it receives from the source to all outgoing interfaces
enabled to receive multicast data. Yet, once a router has determined the location of its source, an SPT can be created,
eliminating hops between the source and the destination, such as the RP. This can be configured by the Switch
administrator by setting the multicast data rate threshold. Once the threshold has been passed, the data path will
switch to the SPT. Therefore, a closer link can be created between the source and destination, eliminating hops
previously used and shortening the time a multicast packet is sent from the source to its final destination.
Register and Register-stop Messages
Multicast sources do not always join the intended receiver group. The first hop router (DR) can send multicast data
without being the member of a group or having a designated source, which essentially means it has no information
about how to relay this information to the RP distribution tree. This problem is alleviated through Register and
Register-Stop messages. The first multicast packet received by the DR is encapsulated and sent on to the RP, which
in turn removes the encapsulation and sends the packet down the RP distribution tree. When the route has been
established, a SPT can be created to directly connect routers to the source, or the multicast traffic can flow from the
DR to the RP. When the latter occurs, the same packet may be sent twice, one type encapsulated, one not. The RP
will detect this flaw and then return a Register-stop message to the DR, requesting it to discontinue sending
encapsulated packets.
Assert Messages
At times in the PIM-SM enabled network, parallel paths are created from source to receiver, meaning some receivers
will receive the same multicast packets twice. To improve this situation, Assert messages are sent from the receiving
device to both multicast sources to determine which single router will send the receiver the necessary multicast data.
The source with the shortest metric (hop count) will be elected as the primary multicast source. This metric value is
included within the Assert message.
PIM-SSM
The Source Specific Multicast (SSM) feature is an extension of IP multicast where datagram traffic is forwarded to
receivers from only the multicast sources to which the receivers have explicitly joined. For multicast groups in the SSM
range, only source-specific multicast distribution trees (no shared trees) can be created.
The Internet Assigned Numbers Authority (IANA) has reserved the address range from 232.0.0.0 to 232.255.255.255
for SSM applications and protocols. The Switch allows SSM configuration for an arbitrary subset of the IP multicast
address range from 224.0.0.0 to 239.255.255.255.
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